MEDICAL PHYSICS INTERNATIONAL Journal, Special Issue, History of Medical Physics 4, 2020 THE MANY STEPS AND EVOLUTION IN THE DEVELOPMENT OF COMPUTED TOMOGRAPHY TECHNOLOGY AND IMAGING METHODS, THE QUEST FOR ENHANCED VISIBILITY The First Fifty Years Perry Sprawls Emory University and Sprawls Educational Foundation, www.sprawls.org. USA • Abstract— Computed Tomography (CT) is considered the second most significant contribution of physics and technology to medicine following Roentgen’s discovery and introduction of x-ray imaging, radiography. CT is an x-ray imaging process but greatly extends the scope of structures, functions, and conditions within a human body that can be visualized for medical diagnosis. Compared to radiography CT provides two major advantages, It is a tomographic method that provides close viewing (within millimeters) of objects and locations within a human body without interference from overlying body sections. The second value is its high contrast sensitivity, the ability to produce visible contrast among different soft tissues, even when enclosed within a dense bony skull. The production of CT images is a sequence of two distinct phases. The first is scanning an x-ray beam around a patient body and measuring the total attenuation along different pathways through the slice of tissue that is to be imaged. This produces a data set consisting of large number (1000s) of measurements. The second phase is a mathematical process of calculating, generally referred to as “reconstructing” a digital image from the acquired data set. Allan Cormack, a physicist, first developed a mathematical process for calculating the distribution of x-ray attenuation values throughout a simulated body section. Godfrey Hounsfield, an engineer on the staff of EMI, developed the technology for scanning and measuring x-ray attenuation along many pathways through a human body. He also developed a mathematical process for reconstructing tomographic images. One of the first, an image of the brain showing a tumor while enclosed in the skull introduced CT to the world as a new and revolutionary diagnostic method. In 1979 Cormack and Hounsfield jointly received the Nobel Prize. EMI manufactured the first CT systems that were limited to imaging the head. Robert Ledley was to soon develop the who body scanner. Two major limitations with the scanners were less than desired image quality and the long time (minutes) required to scan and produce an image, what was to follow were many technical improvements producing different “generations” of CT scanners each generally providing faster data acquisition. Much of this progress was associated with the advances in design of the detectors. The first was a single small detector, followed by multiple detectors and fan-shaped x-ray beams that could make more measurements simultaneously and faster acquisitions. A major advancement was the multi-row detectors that introduced multi-slice imaging. A major and essentially concluding step in the development of general purpose clinical scanners was the invention and development of spiral scanning by Willi Kalender. This rapidly produced volume, rather than slice, data sets and opened up many possibilities for image reconstruction. Along with the many developments to provide improved image quality and faster imaging there were significant advances in radiation dose management and scanners for special clinical applications including dentistry and breast imaging. • Keywords— tomography, reconstruction, digital, scanning, detectors. 351 MEDICAL PHYSICS INTERNATIONAL Journal, Special Issue, History of Medical Physics 4, 2020 I. INTRODUCTION AND OVERVIEW As a clinical medical physicist and educator it has been my opportunity to work with computed tomography from its beginning and on throughout my career. My introduction was a course in the EMI factory where Hounsfield developed the first system in preparation for installing one of the early scanners in my institution, Emory University Hospital in Atlanta. That was to become one of my major projects. Continuing from the first scanner on to the present the capabilities and complexity of CT have expanded and so have the role and responsibilities of medical physicists. As we continue to apply our physics knowledge and experience supporting CT as a highly valuable clinical modality and provide education for future generations there is value in knowing the history leading up to the technology we use today. It was a step-by-step process by many physicists and engineers that is a major part of our heritage. That is the journey that I share with you here. The introduction of computed tomography (CT) for medical imaging in 1976 is sometimes considered as one of the greatest contributions to diagnostic medicine, second to x-ray imaging ( radiography and fluoroscopy) by Roentgen in 1894. Both are physical methods developed by physicists and engineers, with the significance of each being recognized with Nobel Prizes. Both are forms of x-ray imaging; but the difference is how the images are formed. One method simply passes an x- ray beam through a section of a human body and projects shadows. The other (CT) is a much more complex process of two distinct phases. The first phase passes x-radiation through in the plane of slices within a body and measures accumulative attenuations from many different directions. This produces the sets of “scan data.” The next and evolutionary phase is the mathematical “reconstruction” of an image of the slice of body section from the scan data. The fundamentals of mathematical image reconstruction applied to tomographic body sections were developed over the years with several innovative contributions by mathematicians and physicists. However it was the development and availability of digital technology and computers that made it possible for an engineer, Geoffrey Hounsfield, to develop the first computed tomography (CT) to be used for medical diagnosis. The development and clinical application of x-ray CT was to be a major evolution in the field of medicine. Physicians were being presented with a completely new view of the human body, tomograms. There was now the need to learn and teach cross-sectional anatomy. It was now possible to “get in close,” within a few millimeters, and view anatomical features without interference from other body sections. In several ways, scientifically, technically, and medically, x-ray CT was to be an introduction to the many other “computed tomography” imaging modalities that were based on image reconstruction, including MRI, SPECT, and PE--each using computed tomographic imaging but providing visualization of very different tissue and functional characteristics. Our specific interest here is x-ray computed tomography. From its initial development and introduction for medical imaging in 1976 it has continuously evolved with many innovations with the goal of enhanced visualization of structures and conditions within the human body and with considerations for radiation exposure and associated risks--two often opposing goals because several aspects of image quality are directly or indirectly dependent on the quantity of radiation used in the imaging process. 352 MEDICAL PHYSICS INTERNATIONAL Journal, Special Issue, History of Medical Physics 4, 2020 The characteristic of CT that has been a driving force for continuing physics research and technological development is that it is a sequential imaging process requiring a series of x-ray attenuation measurements which is time consuming. A measurement of progress and advancement over the years has been scanning speed and the time required to produce an image. This is significant for several reasons including patient throughput and ability to image with reduced patient motion interference. An associated factor is that image quality, especially detail (spatial resolution) and noise are dependent on the number of measurements (samples) in a scan and data acquisition. It is the combination of increased image quality (clinical visibility), limiting radiation dose, and reduced acquisition time that has resulted from the efforts of many physicists and engineers for now a half century. Those contributions are extensively recorded in the scientific literature. Books and review articles focusing on the history, as identified in the References and Bibliography, provide the details of this major era of medical physics and the associated professions. Our purpose here is not to repeat the many excellent publications, both scientific reports and historical reviews, but to provide a guide and overview through the continuing evolution of computed tomography physics and technology. The Bibliography at the end of this article identifies publications on the history of CT. Two of the most comprehensive are the books, From the Watching of the Shadows by S Webb and Computed Tomography by W Kalender. Here we will consider the vision and motivation of the contributors, the relationship and dependence on other scientific and technological developments, and the efforts to increase the value of CT as a major medical method for diagnosis and guiding therapeutic procedures. The history of computed tomography is not just a series of events along a timeline. It is a comprehensive and dynamic process of expanding and enhancing the clinical value and capabilities of x-ray imaging. It is a continuing step-by-step process of developments and innovations increasing the quality and capabilities of CT as a medical procedure. The steps range from small to large